Method and apparatus for determining the azimuthal orientation of a medical instrument from MR signals

Abstract

A medical apparatus, includes a medical instrument with an acquisition unit for a magnetic resonance signal, the acquisition unit exhibiting a non-constant sensitivity profile in the azimuthal direction relative to an axis of the medical instrument. An azimuthal orientation of the medical apparatus is determined by an evaluation unit from magnetic resonance signals received by the acquisition unit.

1. A medical apparatus, for use with a magnetic resonance imaging system that generates a magnetic resonance image from magnetic resonance signals in an imaging volume, said medical apparatus comprising:
 a medical instrument configured for introduction into said imaging volume, said medical instrument having a longitudinal axis and said medical instrument being non-rotationally symmetric around said longitudinal axis; 
 an acquisition unit attached in a fixed physical relation to said medical instrument, said acquisition unit having a reception sensitivity profile for said magnetic resonance signals that is non-constant in a plane substantially perpendicular to said longitudinal axis, and said acquisition unit receiving said magnetic resonance signals in said imaging volume and generating an output signal therefrom; and 
 an evaluation unit in communication with said acquisition unit and supplied with said output signal from said acquisition unit, said evaluation unit being configured to electronically determine from the magnetic resonance signal received by said acquisition unit, a rotational orientation of said medical instrument in said plane, around said longitudinal axis, by identifying said rotational orientation due to said sensitivity profile being non-constant in said plane. 
 
   
   
     2. A medical apparatus as claimed in  claim 1  wherein said acquisition unit has a reception sensitivity profile that is non-constant in the azimuthal direction relative to a phase of said magnetic resonance signal. 
   
   
     3. A medical apparatus as claimed in  claim 1  wherein said acquisition unit has a reception sensitivity profile that is non-constant in the azimuthal direction relative to an intensity of said magnetic resonance signal. 
   
   
     4. A medical apparatus as claimed in  claim 1  wherein said acquisition unit comprises a co-axial cable having a cable shielding with an opening therein. 
   
   
     5. A medical apparatus as claimed in  claim 4  wherein said shielding has a circumference, and wherein said opening comprises approximately one-half of said circumference. 
   
   
     6. A medical apparatus as claimed in  claim 1  wherein said acquisition unit comprises a coil arrangement. 
   
   
     7. A medical apparatus as claimed in  claim 6  wherein said coil arrangement comprises a dipole coil having two windings each tilted with respect to said axis. 
   
   
     8. A medical apparatus as claimed in  claim 6  wherein said coil arrangement comprises a transversal dipole coil. 
   
   
     9. A medical apparatus as claimed in  claim 1  wherein said medical instrument comprises a reference unit that also receives said magnetic resonance signal, said reference unit having a reception sensitivity profile differing from the reception sensitivity profile of said acquisition unit in at least a range of said azimuthal direction. 
   
   
     10. A medical apparatus as claimed in  claim 9  wherein said reference unit comprises at least one coil. 
   
   
     11. A medical apparatus as claimed in  claim 10  wherein said coil has a constant reception sensitivity profile in said azimuthal direction. 
   
   
     12. A medical apparatus as claimed in  claim 9  wherein said reference unit comprises a first coil and a second coil, said first coil being disposed at a different spacing along said axis from said acquisition unit than said second coil. 
   
   
     13. A medical apparatus as claimed in  claim 1  wherein said medical instrument is a catheter. 
   
   
     14. A medical apparatus as claimed in  claim 13  wherein said catheter has a catheter tip that is angled relative to said axis. 
   
   
     15. A method for determining the azimuthal orientation of a medical instrument, comprising the steps of:
 introducing a medical instrument into an imaging region of a magnetic resonance imaging system, said medical instrument having a longitudinal axis, said medical instrument being non-rotationally symmetric around said longitudinal axis, and said medical instrument carrying an acquisition unit in fixed relation relative to said axis, said acquisition unit having a reception sensitivity profile for said magnetic resonance signals that is non-constant in a plane substantially perpendicular to said longitudinal axis; 
 detecting said magnetic resonance signals with said acquisition unit in generating an output signal therefrom; and 
 from said output signal, electronically determining a rotational orientation of said medical instrument in said plane, around said longitudinal axis, dependent on said reception sensitivity profile of said acquisition unit being non-constant in said plane. 
 
   
   
     16. A method as claimed in  claim 15  wherein said instrument further comprises a reference unit in a fixed spatial relation to said acquisition unit, and comprising the additional steps of:
 detecting said magnetic resonance signals with said reference unit and generating a reference signal therefrom; and 
 from said reference signal and said output signal, electronically determining a position and a non-azimuthal orientation of said axis of said instrument. 
 
   
   
     17. A method as claimed in  claim 15  comprising electronically analyzing a phase of said output signal to determine the azimuthal orientation of the medical instrument. 
   
   
     18. A method as claimed in  claim 15  comprising electronically analyzing an intensity of said output signal to determine the azimuthal orientation of the medical instrument.